A combination of things. I won't even click on a profile if it has an obvious 'fake' picture or a picture not of the person i.e their dog or a cartoon character, if they are ashamed to be on there I don't want to date them. Other turns offs are very bad photos, not a person's looks but ones where they're barely in focus, or in shot again if they can't be bothered to take a half decent picture, which is so easy these days, it makes me think they just aren't putting any effort it. I generally have to be at least slightly attracted to someone to click on a profile, unless they have messaged me first and I'm curious.
Empty profiles or those with few words, again it shows they can't be bothered.
Also evidence or allusions to attitudes I know I won't get on with, i.e sexist jokes about women making sandwiches in the kitchen, if they've answered any questions that reveal them to be homophobic or racist (you'd be surprised what people admit to on sites like OK cupid!).
so I suppose it's a combination of looks and what is revealed about their personality (or apparent lack of it).